1. Field of the Invention
The present invention relates to a ram air turbine for driving an electric generator in a missile, the turbine including a turbine wheel to which ram air is supplied.
A typical characteristic of ballistic missiles is that the velocity along the trajectory may vary greatly depending on the firing angle. Additionally, in certain types of missiles there is a variation in velocity because of a corresponding variation in the discharge velocity of the missile. Thus, the total variation in velocity for such types of missiles may be rather large.
When using ram air turbine driven generators for continuous supply of electric power in such missiles the said variation in missile velocity may constitute a problem, especially when the ratio between the highest and lowest velocity is greater than 4:1 and the maximum velocity is high. It may for instance be desirable to obtain satisfactory rotational speeds of the turbine at missile speeds in the range from 150 to 1.000 m/s. On one hand it is desirable to obtain at the lowest missile velocity an electric power supply which is sufficient for the electric circuits in the missile to function. On the other hand it is desirable to avoid extremely high turbine speeds which may occur at the highest missile speed, since such turbine speeds make heavy demands on the turbine and the generator.
It is therefore desirable for the turbine to reach a certain minimum speed of rotation as soon as possible after launching without the speed of rotation becoming too high at the hihest missile speeds.
2. The Prior Art
Various methods have been suggested in order to solve this problem. Reference is made to U.S. Pat. Nos. 2,701,526, 2,804,824 and 4,161,371, as well as to U.S. Pat. No. 4,267,775, in which the two first mentioned U.S. patent specifications are discussed.
The invention according to U.S. Pat. No. 4,267,775 solves the problem to a large extent without resorting to movable parts, the flow of air being controlled in such a way that the rotational speed will not be too high. However, in cases when an even greater control is desirable it has turned out that solutions are required which are based on movable parts.